Rapid and reliable detection of microorganisms in the blood is among the most important functions of the clinical microbiology laboratory. Several different blood culture systems and approaches are available to laboratories. For example the BACTEC® radiometric and nonradiometric systems (Becton Dickenson Diagnostic Instrument Systems, Sparks, Md.) are often used for this task. For example, the BACTEC® 9240 instrument accommodates up to 240 blood culture vessels and serves as an incubator, agitator, and detection system. Each vessel contains a fluorescent CO2 sensor, and the sensors are monitored on a continuous basis (e.g., every ten minutes). Cultures are recognized as positive by computer algorithms for growth detection based on an increasing rate of change as well as sustained increase in CO2 production rather than by the use of growth index threshold or delta values. The BACTEC® 9240 is completely automated once the vessels have been loaded.
Optimal performance of a blood culture system, such as the BACTEC® 9240, is dependent on collecting the correct amount of blood per sample. Culture of a sample below the optimal level can affect organism recovery based on a decreased probability of obtaining viable organisms from the limited blood volume. Culture of a sample above the optimum level can reduce the recovery of viable organisms by failing to properly dilute or remove inhibitors in the sample or by creating an unfavorable competitive situation with blood competing with any microbes present in the specimen for nutrients such as oxygen or sugar and thereby exceeding the design properties of the culture medium. Blood can also affect the performance of the system by masking the presence of growth when it is present. For example, it is possible for the acceleration of signal used to detect the presence of microorganisms to be homogenized into the blood background signal when too much or too little blood is cultured.
Given the above-background, what are needed in the art are methods for determining the amount of blood in a culture. The ability to determine the amount of blood in a culture in practice would, for example, allow a feedback system on the quality of the blood cultures (including phlebotomy feedback), the ability to identify vessels that are extremely over or under filled during protocol (to warn the staff that quality of the culture is compromised) and to adjust internal growth detection algorithms based on the presence of different levels of blood.